Kaolin clay is a widely used industrial mineral. The clay occurs as an ore from which grit must be removed for virtually all end uses of the clay. The resulting degritted crude kaolin is composed largely of kaolin particles that usually have a wide range of sizes ranging from slimes (finer than 0.3 microns) up to about 10 microns. Frequently discrete mineral impurities such as titania, various ferruginous minerals, mica and nonkaolinitic clays such as bentonite and attapulgite, are present. Such nonkaolin minerals can be removed at least partially by means such as froth flotation, selective flocculation, magnetic separation, bleaching and combination of such steps. The purified kaolin particles are polydisperse, i.e., the size of the particles represent a wide range of sizes. For example, a kaolin that is 100% by weight finer than 2 microns, may contain kaolin particles ranging in size from submicron through 2 microns.
Most uses of purified kaolin require stringent control of the size of the kaolin particles. For example, a #2 coating clay must be about 80% by weight finer than 2 microns. A #1 coating clay must be about 90% by weight finer than 2 microns. A high glossing (#0) grade is nearly 100% by weight finer than 2 micron. Particle size distribution is also controlled for many commercial uses. Thus, two clays may be #1 coating clays as defined by the minus 2 micron parameter. However, a #1 grade with a narrow particle size distribution will generally provide greater opacity and gloss than a #1 grade with a broad particle size distribution. Narrowing of particle size distribution can be achieved by fractionation to reduce oversize and/or desliming to reduce undersize.
Conventional centrifuges, such as Bird solid bowl machines, are widely used to fractionate kaolins. Typically, such centrifuges operate at "g" forces of about 800-1200. Centrifuges that apply greater "g" forces than the Bird machines are needed to produce deslimed kaolins. An examples of a high speed centrifuge is a horizontal three-phase centrifuge, such as one commercially available from Alfa Laval Co. (Greenwood, Ind.). Such centrifuges can be operated in the range of about 1,000 to 10,000 "g" forces, preferably in the range of about 1500-3000. A "g" force of about 5000 is typical. These high speed centrifuges can effect a sharp separation of kaolin particles finer than about 0.3 microns from larger kaolin particles. However, the capital and operating costs are high. Also, these centrifuges are subject to excess wear, especially wear of the nozzles, necessitating frequent repair or replacement.
Most uses of kaolin mineral particles require that the kaolin particles be dispersed or dispersible in water. This is true whether the kaolin particles are intended for use as an intermediate in one or more beneficiation schemes or whether the kaolin particles are intended for a commercial end-use, such as paper coating. Nondispersible kaolin products or by-products have limited commercial value.
It has long been the practice of the industry to fractionate kaolin into kaolin particles of one size range from kaolin particles having a different size prior to and/or subsequent to certain beneficiation treatments carried out with dispersed aqueous pulps. For example, the flotation beneficiation process known as ULTRAFLOTATION requires the use of a fraction of kaolin that is finer than typical degritted kaolin crudes of the type frequently referred to as "coarse" or "soft" crudes. See U.S. Pat. No. 2,990,958. Conventional (Bird) centrifuges can operate effectively to produce typical #1 and #2 fractions as a step prior to carrying out this beneficiation process. On the other hand, some processes practiced in the kaolin industry require or are improved by removing slimes before or after carrying out the unit operation. An example is the use of mechanical means to remove slimes before or after delamination. Reference is made to copending application, U.S. Ser. No. 08/384,973, filed Feb. 7, 1995, (and refiled as U.S. Ser. No. 08/677,758), entitled "DELAMINATED KAOLIN PIGMENT, THEIR PREPARATION AND USE", Behl, et al. When true desliming, i.e., removal of essentially all slimed particles is required, the use of centrifuges operated at very high "g" values was necessary in the past.
In contrast to mechanical systems employed in minerals processing industries to effect separations based on differences in mineral species or size, chemical means are also used in processing industrial minerals. Froth flotation, referred to above, is an example of a chemical system. In selective flocculation, charged inorganic or organic molecules are used to selectively flocculate minerals from each other based on difference in mineral species. This is fundamentally different from separating minerals of one species from minerals of the same species, e.g., the separation of fine kaolin particles from coarser kaolin particles.
A recent example of the use of selective flocculation to separate minerals based on differences in species is U.S. Pat. No. 5,535,890, Behl, et al. This patent relates to a flocculation process especially useful in separating colored mineral impurities, especially titania, from kaolin particles. The processing includes the use of a dispersant to provide a fluid pulp, conventional kaolin froth flotation reagents such as oleic acid, a source of calcium ion, and a high molecular weight anionic polymeric flocculating agent. The impurities settle as a lower flocculated layer from an alkaline pulp and the purified kaolin is recovered as a dispersed fluid suspension which is separated from the sedimented impurities by decantation.
Organic polymers have also been proposed to achieve selective flocculation of kaolins into dispersed and flocculated fractions differing in viscosity. An early example is U.S. Pat. No. 2,569,680, Leek, which utilizes the difference in surface chemistry between some kaolin particles and other kaolin particles giving rise to high viscosity. A more recent example is U.S. Pat. No. 4,334,985 Turner, et al. Turner, et al., seek to remove small amounts of coarse aggregates of kaolin from kaolin particles to improve the viscosity of the remaining kaolin. The processing uses an anionic organic polymer to bring about separations comparable to those achievable with conventional (e.g., Bird) centrifuge. Thus, coarse aggregate are removed as a sediment from a kaolin material and the remaining kaolin, the desired product is recovered as a dispersed slip. Turner, et al., do not fractionate a kaolin into a slimed and nonslimed fractions such as is accomplished using an Alfa Laval separator as the dispersed phase material. Rheology data, in the patent indicate that the flocculated aggregates are not readily dispersible.
All particle sizes referred to herein are determined by a conventional sedimentation technique using a SEDIGRAPH.RTM. 5100 analyzer analysis. The sizes, in microns, are reported as "e.s.d." (equivalent spherical diameter).